The process of wet combustion (or wet oxidation, as it is often called) was developed in Sweden by Cederquist in the late 1940's for processing peat, and by Zimmerman in the United States for processing sewage liquids and black liquors from the wood pulping industry. Patents were granted to both in the early 1950's; and both depended on tubular heat exchangers, high pressure pumps, and particularly on continuous processing and streams of liquids and gases to heat the aqueous liquors to the high temperatures necessary for the combustion to be ignited in the aqueous solution or slurry by the addition of air. Both also passed the hot liquid, after the wet combustion, to the heat exchangers for preheating the original liquid; and both used one or more high pressure pumps to raise the liquid to the maximum pressure of the reaction or combustion zone.
Othmer in U.S. Pat. No. 3,692,634 showed that the expensive and highly corrodable tubular heat exchangers could be replaced by the open condensation of steam, without heat transfer surfaces -- the so-called Vapor Reheat system. This operates a series of stages of multiple flash evaporations of the hot liquid after the web combustion, each flash evaporating stage then supplying steam from its one of the successive multiple flash evaporations to a corresponding one of a multiplicity of open condensation stages in a counter current system. The original liquid was thus heated by the several condensations of steam -- from the corresponding flash evaporations -- to the high temperature appropriate and necessary for the process in question. In some cases, after the preheating operation it was necessary to heat in a prime heater still further with steam or other heat source to bring to the high temperature necessary for the chemical or physical process. In other cases e.g. wet combustion, the reaction itself supplied the heat necessary to make up -- or even much more than make up -- the heat losses in the heat interchanging system, thus no other source of prime heat was necessary.
While the flash evaporation of a liquid or of a batch of liquid is often referred to, it is to be noted that actually only a relatively small amount of the liquid is actually converted to steam. Thus a liquid at the temperature of 250.degree. F and a corresponding pressure may have the pressure above it lessened so that it flash evaporates as it generates steam to help it come to equilibrium with the new pressure. If the controlling temperature is that of open condensing water which establishes a temperature of, say 225.degree. F, and corresponding pressure, then the hot liquid may be cooled a maximum of 250.degree.-225.degree. or 25.degree.. Since the specific heat of water is approximately 1 BTU per pound per .degree. F and the latent heat per pound is roughly 1000 BTU per pound, it follows that about 25/1000 or 0.025 pounds of steam will be generated for every pound of water cooled in this single flash evaporation. Also if the same amount of water (1 pound) was being heated by this condensation, the 25 BTU input would have raised that water 25.degree. F; and its original temperature must have been 225.degree.-25.degree. = 200.degree. F. Many corrections would be needed to refine this simple heat balance, but it is illustrative for the purpose.
Also U.S. Pat. No. 3,692,634 showed that a wet combustion could be controlled so as to oxidize some compound or class of organic compounds which were "softer" or relatively easily oxidized while not oxidizing others which are "harder" or more difficult to oxidize. This is referred to as a partial oxidation or a partial combustion.
U.S. Pat. No. 3,692,634 described other processes, involving physical and/or chemical operations at high temperatures with aqueous liquids -- solutions and/or slurries -- which can be accomplished using Vapor Reheat heat exchanging which is operated continuously. This heat exchange system with heat transfer surfaces heats the cold liquid and its accompanying solid constituents - either dissolved or suspended - and cools the hot liquid. However, U.S. Pat. No. 3,692,634 requires pumps operating between the condensing side of each of the condensing stages to pump the liquid to the higher pressure of the next higher stage. These pumps taken together bring the liquid to the highest pressure in the system. Again there is the capital expense of the pumps and the expense of their operation and maintenance.
The process of U.S. Pat. No. 3,692,634 demonstrated that it could be used for the hydrolysis of carbohydrate materials, also the wet combustion of sewage liquids, and of black liquors from wood pulping operations. With simultaneous treatment with an alkaline material as sodium or calcium carbonate or calcium hydroxide, the wet combustion gave alkaline acetates. The production and revivification of powdered activated carbons was also disclosed, also the extraction of water from sea water with solvents having different mutual solubilities at higher and lower temperatures, and other processes wherein a physical operation or a chemical reaction takes place at an elevated temperature due to the addition of another material. The present improvement operating with batches of liquid allows the accomplishment of each of these processes with additional advantages when operated as described hereinafter.
The novel advantages of this new batch process also are noted in those cases where, for particular reasons, it may be desired to accomplish the condensation of the vapors of this batch heat transfer operation in a closed condensing system. This will produce distilled water, will allow a more concentrated liquid for the reaction and will produce a higher concentration in the effluent or spent liquor. This may be particularly valuable with those processes operating over a large temperature range, e.g. wet combustion because of the large amount of steam formed in the several flash evaporations, with corresponding losses of liquid and changes in its concentration, and of the large amount of distilled water which can be produced.
Particularly in those cases where a desired chemical reaction is to be completed, the present batch process allows a precise control which is difficult or impossible with the continuous processes. Thus, in the partial wet combustion in the presence of an alkali of organic materials, as sewage sludges and wood pulping black liquors, the concentration of acetate salts increases. This is because acetic acid is formed as an intermediate product in the partial wet combustion of many organic materials. At the maximum concentration, the wet combustion can be stopped, the liquors are removed and treated with sulfuric acid to give free acetic acid; and this can be recovered, by known methods, e.g. those of U.S. Pat. No. 2,878,283.
The wet combustion of carbohydrate material -- i.e. sugars, starches, celluloses, etc. -- gives acetic acid as such as one of the intermediary products, and it may be removed in a dilute form from surface condensers in the preheating vessels; or if alkali is present, as the alkaline acetate. In either case it may ultimately be recovered as glacial acetic acid by known processes. The intermediary steps in the breakdown and oxidation of the large carbohydrate molecule may only be surmised, one sequence may be via alcohol, then aldehyde, as in the chemistry of other processes. Ethanol itself in a wet oxidation in aqueous solution has been found to give acetic acid as such or as an acetate salt in the presence of an alkali.
In other treatments by partial wet combustion by this method, of waste liquids such as those from the pulp industry, the addition of oxides, hydroxides, or carbonates of calcium, magnesium, or iron, results in insoluble salts or complexes of acids of higher molecular weights than acetic acid; and by stopping of the wet combustion short of burning out the carbon and hydrogen of these salts or complexes, and filtering them out of the spent liquors, water insoluble materials of value may be obtained. These materials are difficult or almost impossible of analysis but may be used as a cheap extender of asphalts in many of their uses.
Before, during and/or after the processing of a batch, the vessel it is in may have to be vented. If at a superatmospheric pressure non-condensible gases may generate power or go to the atmosphere, if at lower pressures, means for subatmospheric venting through a corresponding vacuum are provided.